Such lithographic methods are used for producing various metal microcomponents. These methods are of special importance for producing microcoils for electric micromotors. In order to increase the performance of micromotors on one hand and reduce the overall height on the other hand, these microcoils must have a large fill factor or a high structural density and small thickness. Furthermore, these microcomponents must be self-contained.
The known coils for micromotors do not meet these requirements.
DE 41 05 999 A1 describes a film coil that is used in a supra-conducting apparatus. These are layered film coils in which a printed circuit board is applied to an isolating substrate with the board being etched so that spiral-shaped printed circuit boards are produced. Possible materials that may be used are copper or aluminum. However, etching is disadvantageous in that the microstructures are undercut so that their aspect ratio that usually is around ≦1 is significantly restricted. With the current technology only structures with relatively low heights may be produced. Typical heights for the thin conductor technology usually do not exceed 15 μm while the lower limit for the strip conductor width is around 50 μm with a minimum strip conductor distance of 60 μm. Due to the relatively large distances of the coil layers, the fill factor is relatively low as well.
From Proc. SPIE Vol. 3680B-65 Paris, France, Mar. 30 through Apr. 1, 1999 “Micromachining and Microfabrication”, title “Design and realization of a penny-shaped micromotor” by M. Nienhaus et al. it is known to first apply a copper start layer on a substrate material with resist material being applied to the copper layer via a bonding layer. This resist material is structured by means of a mask and UV radiation and then the resist structures are filled with metal by means of a galvanic method. The resist material between the metal structures is removed so that only the metal structures are left. A coil produced in this manner is not self-contained and must be applied to a support foil together with additional coils.
The resist material used is SU-8 Resistmaterial (Shell Chemical commercial name), which is a cross-linkable polymer that is described in J. Micromechanics, Microengineering 7(1997), pp. 121–124, for example. It is an epoxy derivative of bisphenol-novolak that is already being used in the production of microstructures. This material above all is characterized in that it is possible to produce structures with high aspect ratios. Due to the cross-linking during the exposition the refraction index of this material changes so that structures with conductive properties are produced from the resist material. The exposition by means of masks therefore produces vertical walls that are maintained when the unexposed zones are etched away.
The known method requires that the SU-8-Resistmaterial be removed. However, the removal is extremely difficult because the material is cross-linked. It requires special solvents that, in principle however, only cause the resist material to swell up. Removal is only possible if the metal structures have a sufficiently large distance of typically >100 μm so that the solvent may develop its full potential between the metal structures. This means the structural density of microcomponents produced in this manner is very limited. Since the resist material also must be removed when the microcomponents are subdivided, this method is quite labor-intensive.
Microcomponents made of plastic, e.g. of SU-8, may be produced by means of lithographic methods in which first a negative metal mold is produced by means of galvanic molding, for example. This negative mold then is used as a molding tool for an injection molding process for producing the plastic micro-component. This method that is described in EP 0 851 295 A1, is very labor intensive due to the numerous individual process steps. Furthermore, the production of microcomponents with undercuts is not possible.